Lithium/oxyhalide electrochemical cell systems such as Li/SOCl.sub.2 (lithium/thionyl chloride) or Li/SO.sub.2 Cl.sub.2 (lithium/sulfuryl chloride) are primary electrochemical cells having a high energy density and a relatively long operating life. One of the potential practical uses for these systems is as a power source in applications requiring long battery life such as, for example, automatic meter reading systems. Typically, the electrical current consumption of such systems includes a sustained low background current of several microampers and intermittent short current pulses having an amplitude of several tens to several hundreds of milliampers and a duration in the milliseconds range.
Unfortunately, during storage at open circuit conditions or under low background currents the lithium anode of lithium/oxyhalide cell systems becomes passivated by a film which substantially reduces the operating voltage of the battery. As a result, during high current pulses, cell voltage drops to a low level. This low voltage problem can be partially overcome by adding an organic compound such as polyvinyl chloride or an inorganic compound such as SO.sub.3 to the cell solution for modifying the passive film to increase its conductivity. However, such additives do not completely solve the passivation problem for the full cell's life span and after a few months the cells develop a similar passivation leading to low cell voltage.
Another possible solution is to increase the surface area of the cell electrode. For example, the low surface area bobbin type design can be replaced with a "jelly roll" type design having a high electrode surface area. Unfortunately, This approach provides only a partial solution to the problem since a one to two years old cell having the jelly roll type design develops a similar passivation and low voltage problem.
Another disadvantage of the jelly roll design oxyhalide batteries is that under certain conditions such as short circuit, compression or nail penetration the cell may explode.
It is well known in the prior art that a capacitor can be connected in parallel with a primary electrochemical cell such as a Li/oxyhalide cell to form the circuit illustrated in FIG. 1 to which reference is now made. FIG. 1 is a schematic electrical circuit diagram illustrating a prior art electrical circuit 2 including a primary electrochemical cell 4 connected in parallel with a capacitor 6. This arrangement can somewhat reduce the voltage drop which occurs when current is drawn from the circuit 2. Typically, the capacitor 6 is charged by the primary electrochemical cell 4 until the voltage across the capacitor 6 is equal to the voltage across the primary electrochemical cell 4. The electrochemical cell 4 will then need to supply a small current required to compensate for the capacitor leakage. When the circuit 2 needs to apply a large current pulse across a load (not shown) connected to the terminals 8 and 9, part of the current will be initially supplied by the capacitor 6, reducing the amount of current drawn from the electrochemical cell 4 and thus at least initially reducing the resulting decrease in the voltage across the electrochemical cell 4.
Unfortunately, this approach has only limited applications because to sustain an acceptable voltage level for extended durations the circuit 2 will require capacitor 6 to have a very large capacitance value. Typically, such large capacitors will be prohibitively bulky and expensive for many types of applications. Moreover, the larger the capacitor 6, the larger will be the rate of charge leakage from the capacitor 6, thus, undesirably increasing the discharging rate of the electrochemical cell 4.
Using a "super-capacitor" as the capacitor 6, for example a model FEOH474Z super capacitor, commercially available from NEC corporation, Japan, does not solve the problem since such capacitors have a very high impedance value, limiting the magnitude of the instantaneous current that can be supplied by such super-capacitors. Moreover, this super capacitor has a relatively high leakage current which undesirably increases the discharging rate of the electrochemical cell.